Chlorosilanes such as trichlorosilane and silicon tetrachloride can be prepared, for example, by reacting crude silicon with chlorine or hydrogen chloride. In the case of an industrial scale preparation, types of crude silicon whose silicon content is 85% by weight or more are usually employed as starting materials. Other constituents of crude silicon are mainly iron, aluminum, calcium and titanium, which are converted into their chlorides during the reaction with chlorine or hydrogen chloride. In addition to these metal chlorides, high-boiling compounds such as hexachlorodisiloxane and pentachlorodisiloxane are also produced.
Customarily, these residues are coarsely separated from the chlorosilanes by distillation. Depending upon the distillation conditions, the distillation residue is left behind in the form of a suspension or as a solid, which must be subjected to a special processing procedure prior to environmentally compatible disposal thereof.
The distillation of the chlorosilanes is carried out as completely as possible, because any chlorosilanes which remain in the distillation residue can no longer be converted into useful products and therefore represent a loss in value. A substantially evaporated distillation residue has a typical composition of about 80% by weight of aluminum and iron chlorides, 16% by weight of hexachlorodisiloxane, pentachlorodisiloxane and titanium tetrachloride, and 4% by weight of silicon tetrachloride.
German Auslegeschrift No. 21 61 641 discloses a method wherein the distillation residue from a chlorosilane production is poured into a heatable paddle dryer, chlorosilanes are driven off at the appropriate temperature accompanied by agitation of the dryer contents, and after drying of the solids is complete, steam is blown into the paddle dryer to effect the hydrolysis. The disadvantage of such a batch process is that the hydrolysis residue cannot be completely removed from the mixer after the hydrolysis is complete, so that hydrolysis residue remains on the walls and the paddles, which reacts with a new charge of chlorosilane-containing suspension to form hard, crust-like deposits which gradually clog the hydrolysis reactor and greatly impair the functioning of the shut-off devices which are required for the drying and hydrolysis process steps.
German Patent No. 36 42 285 discloses a batch process wherein the drying and hydrolysis steps are performed in two different apparatuses at different temperatures, where drying takes place at a lower temperature and the hydrolysis is effected at a higher temperature. In order to minimize the proportion of aluminum chloride in the distillate during drying, the drying temperature should not significantly exceed 130.degree. to 140.degree. C. On the other hand, effective hydrolysis requires temperatures of 170.degree. C. and higher, since increasing temperatures naturally lead to better hydrolysis reactions.
The process described in this German patent also has certain drawbacks, because the jump in temperature which the treated residue undergoes when the drying step turns into the hydrolysis step leads to flash evaporation of a portion of the residue, whereby the hydrolysis reaction takes place partly in the gaseous phase and the hydrolysis products are carried out of the reactor together with the exhaust gas due to their low weight. The separation of the solids from the exhaust gas stream containing hydrogen chloride and unconverted steam is complicated. If dust filters are used, they must be specially equipped to avoid condensation of hydrochloric acid, and the service life of the filter cloth is short due to a tendency to clog. If, on the other hand, scrubbers are used, the solid must be separated mechanically from the hydrochloric acid and must then be subjected to an after-treatment, for example drying, because it cannot be disposed of in landfills in its untreated form, namely moistened with hydrochloric acid.
The procedure described in German Auslegeschrift No. 21 61 641 also leads to a high discharge of solids by way of the gaseous phase which leaves the hydrolysis.